Multilayer optical compensator, liquid crystal display, and process

ABSTRACT

A multilayer compensator includes one or more polymeric first layers and one or more polymeric second layers. The first layers comprise a polymer having an out-of-plane (Δn th ) birefringence not more negative than −0.01 or not more positive than +0.01. The second layers comprise an amorphous polymer having an out-of-plane birefringence more negative than −0.01 or more positive than +0.01. An overall in-plane retardation (R in ) of the multilayer compensator is greater than 20 nm and the out-of-plane retardation (R th ) of the multilayer compensator is more negative than −20 nm or more positive than +20 nm. The in-plane retardation (Rin) of the one or more first layers is 30% or less of the overall in-plane retardation (Rin) of the multilayer compensator.

FIELD OF THE INVENTION

The present invention relates to a multilayer optical compensator forliquid crystal displays. The invention also relates to a process formaking such a compensator and liquid crystal displays using thecompensator.

BACKGROUND OF THE INVENTION

Liquid crystals are widely used for electronic displays. In thesedisplay systems, a liquid crystal cell is typically situated between apair of polarizer and analyzers. An incident light polarized by thepolarizer passes through a liquid crystal cell and is affected by themolecular orientation of the liquid crystal, which can be altered by theapplication of a voltage across the cell. The altered light goes intothe analyzer. By employing this principle, the transmission of lightfrom an external source including ambient light, can be controlled. Theenergy required to achieve this control is generally much less thanrequired for the luminescent materials used in other display types suchas cathode ray tubes (CRT). Accordingly, liquid crystal technology isused for a number of electronic imaging devices, including but notlimited to digital watches, calculators, portable computers, andelectronic games for which light-weight, low-power consumption andlong-operating life are important features.

Contrast, color reproduction, and stable gray scale intensities areimportant quality attributes for electronic displays, which employliquid crystal technology. The primary factor limiting the contrast of aliquid crystal display (LCD) is the propensity for light to “leak”through liquid crystal elements or cells, which are in the dark or“black” pixel state. Furthermore, the leakage and hence contrast of aliquid crystal display are also dependent on the direction from whichthe display screen is viewed. Typically the optimum contrast is observedonly within a narrow viewing angle range centered about the normalincidence to the display and falls off rapidly as the viewing directiondeviates from the display normal. In color displays, the leakage problemnot only degrades the contrast but also causes color or hue shifts withan associated degradation of color reproduction.

LCDs are quickly replacing CRTs as monitors for desktop computers andother office or household appliances. It is also expected that thenumber of LCD television monitors with a larger screen size will sharplyincrease in the near future. However, unless problems of viewing angledependence such as hue shift, degradation in contrast, and an inversionof brightness are solved, LCD's application as a replacement of thetraditional CRT will be limited.

A Vertically-Aligned liquid crystal display (VA-LCD) offers an extremelyhigh contrast ratio for normal incident light. FIG. 2A and FIG. 2B arethe schematics of VA liquid crystal cell in OFF 201 and ON 203 states.In its OFF state, the liquid crystal optic axis 205 is almostperpendicular to the substrate 207, FIG. 2A. With an applied voltage,the optic axis 205 is tilted away from the cell normal, FIG. 2B. In theOFF state, light does not see the birefringence in the normal direction209, giving the dark state that is close to that of orthogonally crossedpolarizers. However, obliquely propagated light 211 picks up retardationgiving light leakage. This results in a poor contrast ratio in someviewing angle range.

A bend aligned nematic liquid crystal display, also referred as anOptically Compensated Bend Liquid Crystal Display (OCB-LCD) uses anematic liquid crystal cell based on the symmetric bend state. In itsactual operation, the brightness of the display using the bend alignednematic liquid crystal cell is controlled by an applied voltage or fieldthat leads to a different degree in the bend orientation within the cellas shown in FIG. 3A (OFF) 301 and FIG. 3B (ON) 303. In both states, theliquid crystal optic axis 305 takes symmetric bend state around the cellmiddle plane 307. In the On state, the optic axis becomes substantiallyperpendicular to the cell plane except near the cell substrates 309. OCBmode offers faster response speed that is suitable to the liquid crystaldisplay television (LCD-TV) application. It also has advantages inviewing angle characteristic (VAC) over conventional displays, such asTwisted Nematic liquid crystal display (TN-LCD).

The above-mentioned two modes, due to their superiority over theconventional TN-LCD, are expected to dominate the high-end applicationsuch as LCD-TV. However, practical applications of both OCB and VA-LCDsrequire optical compensating means to optimize the VAC. In both modes,due to the birefringence of liquid crystal and crossed polarizer, VACsuffers deterioration in contrast when the displays are viewed fromoblique angles. Use of biaxial films have been suggested to compensatethe OCB (U.S. Pat. No. 6,108,058) and VA (JP1999-95208) LCDs. In bothmodes, liquid crystals align sufficiently perpendicular to the plane ofthe cell in ON(OCB) or OFF (VA) states. This state gives positiveR_(th), thus the compensation films have to have sufficiently largenegative R_(th) for satisfactory optical compensation. The need for abiaxial film with a large Rth is also common for Super Twisted NematicLiquid Crystal Display (STN-LCD).

Several methods of manufacturing biaxial films with sufficient negativevalue of R_(th) suitable for compensating LCD modes such as OCB, VA andSTN have been suggested.

U.S. 2001/0026338 discloses a use of retardation increasing agent incombination with triacetylcellulose (TAC). The retardation-increasingagent is chosen from aromatic compounds having at least two benzenerings. By stretching agent doped TAC, one can generate both R_(th) andR_(in). The problems with this method is that the amount of the dopingagent. To generate the desired effects of increasing R_(th) and R_(in),the necessary amount of agent can be high enough to cause unwantedcoloration, or movement (diffusion) of the agent into other layers inthe LCD with a resulting loss of R_(th) and R_(in) and undesiredchemistry in these adjacent layers. With this method, it is difficult tocontrol the values of R_(th) and R_(in) independently.

Sasaki et al. proposes (US2003/0086033) the use of cholesteric liquidcrystal disposed on the positively birefringent thermoplastic substrate.The pitch of the cholesteric liquid crystal (CHLC) is shorter than thewavelength of the visible light, thus properly aligned CHLC exhibitsform birefringence giving negative R_(th). R_(in) is controlled byadjusting the stretching amount of the thermoplastic substrate. Themethod enables one to adjust R_(th) and R_(in) separately. However, theuse of short pitch CHLC not only makes the manufacturing cost high butalso complicates the processing due to the alignment procedure.

JP2002-210766 discloses the use of propionyl or butyryl substituted TAC.They show higher birefringence than ordinary TAC. Thus, by biaxiallystretching the substituted TAC film, one generates R_(in) and R_(th).The method does not require any additional coating or layer but itsuffers a difficulty of independent control of R_(in) and R_(th).

Thus, it is a problem to be solved to provide a multilayer opticalcompensator with independently controlled R_(th) and R_(in) that can bereadily manufactured.

SUMMARY OF THE INVENTION

The invention provides a multilayer compensator that includes one ormore polymeric first layers and one or more polymeric second layers. Thefirst layers comprise a polymer having an out-of-plane (Δn_(th))birefringence not more negative than −0.01 or not more positive than+0.01. The second layers comprise an amorphous polymer having anout-of-plane birefringence more negative than −0.01 or more positivethan +0.01. An overall in-plane retardation (R_(in)) of the multilayercompensator is greater than 20 nm and the out-of-plane retardation(R_(th)) of the multilayer compensator is more negative than −20 nm ormore positive than +20 nm. The in-plane retardation (Rin) of the one ormore first layers is 30% or less of the overall in-plane retardation(Rin) of the multilayer compensator.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a view of a typical layer with thickness d and x-y-zcoordinate system attached to the layer.

FIG. 2A and FIG. 2B are schematics showing, respectively, the typical ONand OFF state of the VA liquid crystal cell.

FIG. 3A and FIG. 3B are schematics showing, respectively, the typical ONand OFF states of the OCB liquid crystal cell.

FIG. 4A, FIG. 4B and FIG. 4C are elevation schematics of the multilayeroptical compensator of the invention.

FIG. 5A, FIG. 5B and FIG. 5C are schematics of a liquid crystal displaywith multilayer optical compensators of the invention.

FIG. 6A illustrates a wide-angle X-ray diffraction pattern for thetransmission mode of a highly ordered, non-amorphous material, and FIG.6B is a wide-angle X-ray diffraction pattern for the transmission modeof an amorphous polymer of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to the description herein:

Optic axis refers to the direction in which propagating light does notsee birefringence.

ON and OFF state refers to the state with and without applied voltage tothe liquid crystal cell.

In-plane retardation, R_(in), of a layer 101 shown in FIG. 1 is aquantity defined by (nx−ny)d, where nx and ny are indices of refractionin the direction of x and y. The x axis is taken as a direction ofmaximum index of refraction in the x-y plane and the y direction isperpendicular to the x axis. Thus R_(in) will always be a positivequantity. The x-y plane is parallel to the plane 103 of the layer. d isa thickness of the layer in the z-direction. The quantity (nx-ny) isreferred to as in-plane birefringence, Δn_(in). It also will always havepositive values. The values of Δn_(in) and R_(in) hereafter are given atwavelength λ=550 nm.

Out of-plane retardation, R_(th), of a layer 101 shown in FIG. 1,herein, is a quantity defined by [nz−(nx+ny)/2]d. nz is the index ofrefraction in z-direction. The quantity [nz−(nx+ny)/2] is referred to asout-of-plane birefringence, Δn_(th). If nz>(nx+ny)/2, Δn_(th) ispositive, thus the corresponding R_(th) is also positive. Ifnz<(nx+ny)/2, Δn_(th) is negative and R_(th) is also negative. Thevalues of Δn_(th) and R_(th) hereafter are given at λ=550 nm.

Amorphous means a lack of long-range order. Thus an amorphous polymerdoes not show long-range order as measured by techniques such as X-raydiffraction. This is demonstrated, by example only, by the contrastinggraphic characteristics illustrated in FIGS. 6A and 6B. FIG. 6Aillustrates a wide-angle X-ray diffraction pattern (transmission mode)of a rigid rod polymer, specifically a(BPDA-TFNB)_(0.5)-(PMDA-TFMB)_(0.5) polyimide as referenced in U.S. Pat.No. 5,344,916. FIG. 6B is a wide-angle X-ray diffraction pattern(transmission mode) of an amorphous polymer of the present invention[poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate].

Chromophore means an atom or group of atoms that serve as a unit inlight adsorption. (Modern Molecular Photochemistry Nicholas J. TurroEditor, Benjamin/Cummings Publishing Co., Menlo Park, Calif. (1978) Pg77). Typical chromophore groups include vinyl, carbonyl, amide, imide,ester, carbonate, aromatic (i.e. heteroaromatic or carbocylic aromaticsuch as phenyl, naphthyl, biphenyl, thiophene, bisphenol), sulfone, andazo or combinations of these groups.

Non-visible chromophore means a chromophore that has an absorptionmaximum outside the range of 400–700 nm.

Contiguous means that articles are in contact with each other. In twocontiguous layers, one layer is in direct contact with the other. Thus,if a polymer layer is formed on the substrate by coating, the substrateand the polymer layers are contiguous.

Commonly assigned U.S. patent application Ser. No. 10/631,152, filedJul. 31, 2003, is incorporated herein by reference. In that application,a multilayer optical compensator is disclosed in which at least oneembodiment thereof is characterized by the provision of an amorphouspolymer coated onto the surface of a previously stretched polymersupport layer. The support layer is stretched to generate an in-planeretardation that is greater than 20 nm.

As explained herein, the present invention is at least partiallycharacterized by simultaneous stretching of both (or all) layers of themultilayer optical compensator after the amorphous polymer layer hasbeen coated onto the surface of the polymer support. The stretching cantake place while the compensator is in a “wet” state, i.e., afterco-casting (or coating) of the layers and prior to (or concurrentlywith) drying of the amorphous polymer. Alternately, or in addition,“dry” stretching can occur after the multilayer compensator has beencast and the amorphous polymer dried. Stretching can occur in atransverse direction, i.e., in a direction coincident with a castingdirection of the film. Alternately, or in addition, stretching can occurin a direction perpendicular the transverse direction. Also alternately,or in addition, stretching can occur obliquely relative to thetransverse direction (i.e. in a diagonal fashion).

In various liquid crystal displays, it is desirable to modify thebirefringence of polarizer stack layers, to optimize the viewing anglefor the complete screen system. The manufacturing methods of embodimentsof the present invention, in combination with specific polymers, allow abasic sheet of triacetylcellulose (TAC) to be modified by a second layer(or co-cast) of amorphous polymer. The thickness of the TAC and thesecond layer polymer can be varied to provide a “tunable” package ofoptical properties. In wet-stretching, stresses applied to the sheetduring manufacturing can control the in-plane (x, y) retardation and thethickness of the second layer polymer can control the out-of-planeretardation. Likewise, in dry-stretching, stresses applied to the sheetafter manufacturing can control the in-plane (x, y) retardation and thethickness of the second layer polymer can control the out-of-planeretardation. This application of amorphous polymers can result in asimple way to create a useful sheet in a cost effective manner.

The multilayer optical compensator may be realized by the use of twoextrusion hoppers intimately stacked on top of each other. In this case,the two polymer solutions meet at the mated die lips of the stackedhoppers. Co-casting is a laminar layering of two polymers in a singledie cavity. The flow characteristics and polymer viscosities arecontrolled with a feed block, to form two distinct layers in a singledie. This operation could also be carried out in two independent hoppersonto the same casting surface. The object is to form the TAC layer(mated to the casting surface) and the second layer polymer (riding ontop of the TAC) at the same time on the casting surface. This leads tooptimum adhesion between the polymers. An alternative is to cast a thirdadhesion layer between the TAC and the second layer, if superioradhesion is desired.

In the experiments as explained in more detail below, four, second layerpolymers were co-cast onto TAC (typical 2.86 acetyl substitution,220,000 M.W. polymer). All of the polymers were dissolved in a methylenechloride or methylene chloride and methanol solutions. The multilayeredoptical compensator was produced at nearly 3.1 mils (80 microns total).The machine line speed was varied from 4 to 6 ft/min. This provides acasting surface drying time of 3 to 4 minutes. At the end of the castingsurface the curing web is stripped from the (highly polished) castingsurface and fed to edge restraint belts. The edge belts are two endlessbelts, which are brought together to form a serpentine path, with thedrying film caught in the nip between the two belts. These belts aredescribed in U.S. Pat. No. 6,152,345 and U.S. Pat. No. 6,108,930, thecontents of which are incorporated herein by reference.

When the wet (significant amounts of solvent present) sheet is in theedge belts, heated drying air is blown at the sheet from both sides. Theair is forced at high temperature and high velocity, to impart rapidheating and drying. If the forced air drying is rapid and temperaturesdo not exceed the Tg (of the sheet and solvent combination) transversestresses can be created to neutralize the machine direction stressesimparted at sheet stripping, or increased beyond that to create atransverse orientation in the two layer sheet. This is not tentering inthe intentional, active stretching sense, but merely the restraining ofshrinkage forces as the polymer sheet dries. It shall be referred to as“passive tentering”. If the heating is applied with sufficient energy,the sheet can be taken above Tg (of the solvent and polymer mixed) andthe drying and stripping stresses can be relaxed out. By using thismethod the in-plane stresses and retardations can be manipulated inmagnitude and orientation.

The out of plane retardation (Rth) of an 80 micron TAC sheet varies fromapproximately −80 nm to an annealed value of about −40 nm. The TAC Rthcan be manipulated by casting surface time and temperature in therestrained heating section.

The second layer of amorphous polymer requires rapid drying to retainits birefringence. The second layer dries rapidly from a volatilesolvent on top of the TAC layer. The solvent from the drying TAC sheetdoes not soften the second layer sufficiently to allow relaxation of themolecules. The thickness of the second layer polymer can be varied tocontrol the optical properties of the multilayered compensator. The Rinof the second layer amorphous polymer can be manipulated by restraintand temperature as described before (for TAC).

Table A below show the results of experiments for examining thebirefringence of optical compensators obtained by co-casting and wetstretching in accordance with embodiments of the present invention. Thefirst sample was a TAC layer only, with no second layer polymer. Theremaining samples each included a second layer polymer on an underlyingTAC layer. In all samples, the TAC layer was formed from a polymersolution of 18.7% wt % TAC, 73.2 wt % methylene chloride and 8.1 wt %methanol.

Table A shows the thickness of the underlying TAC and the thickness ofthe second layer polymer for each of the samples. Each of the sampleswas obtained by placing the samples while still wet into edge restraintbelts and applying plenum heat. The belts resist shrinkage and providewet passive tentering in the transverse direction. The air flowtemperatures of samples are also shown. The width of each sample as castand the width of each sample after wet passive tentering were measuredto calculate the approximate degree (%) of transverse stretch.

Table A shows the resultant in-plane and out-of-plane retardation ofeach sample. These retardations were measured with an ellipsometer(model M2000V, J. A. Woollam Co.) at 550 nm wavelength. As is apparentfrom these results, the magnitude of in-plane and out-of-planeretardation correlates to the degree of stretch and the thickness of thesecond layer.

-   -   where x=93, y=7    -   and a=70, b=30

Poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate Polymer A

Poly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalatePolymer B

TABLE A Second First Layer Second Layer Air Flow Layer (TAC) ThicknessThickness Temperature % Stretch = R_(th) R_(in) Polymer (μm) (μm) (° C.)% Extension (nm) (nm) None 70.5 0 65 0 −50 2 Polymer A 71.1 5.7 65 0.7−67 20 Polymer A 68.6 10.2 65 3.2 −63 34 Polymer A 70.5 12.7 93 4.7 −9554 Polymer A 71.1 19.0 121 8.6 −107 126 Polymer B 61.9 2.9 65 2.7 −59 6Polymer B 62.2 14.0 93 5.0 −78 8 Polymer B 62.2 18.4 121 6.1 −96 23

It has also been found by the inventors that stretching (“activetentering”) of an already dried multilayer optical compensator (6 μm ofpolymer C on 1 μm of bovine gelatin on 80 μm, of TAC) produced desirableamounts of in-plane anisotropy.

-   -   where x=90, y=10    -   and a=70, b=30

Poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate Polymer C

This in-plane anisotropy was achieved at convenient temperatures and atvery low extensions (2 to 12%). Table B below shows the impact that %extension and temperature had on out-of-plane and in-plane retardationof a multilayer optical compensator having negative out-of-planebirefringence. These retardations were measured with an ellipsometer(model M2000V, J.A. Woollam Co.) at 550 nm wavelength. Two first layers(bovine gelatin and TAC) were used for this example. The bovine gelatinserved as a curl control layer. It was noted that adhesion of the secondlayer and the gelatin layer to the TAC layer was much improved after theheating and stretching. In addition, it is believe that such amultilayer compensator as in this example would have enhanced durabilityin regards to loss of Rin and Rth after aging such a compensator inconditions such as 1000 hours at 60° C. and 90% relative humidity

TABLE B % Extension = % Stretch Temp. (° C.) Rth (nm) Rin (nm) 0 roomtemp. −244 2 2 145 −230 15 5 145 −222 22 7.5 145 −219 29 10 145 −232 680 room temp. −244 2 2 135 −213 2 5 135 −230 39 7.5 135 −244 50 10 135−262 65

Table C below shows the impact that % extension and temperature had onout-of-plane and in-plane retardation of a multilayer opticalcompensator (3.5 μm of polymer C on 1 μm of bovine gelatin on 80 μm ofTAC) having negative out-of-plane birefringence. These retardations weremeasured with an ellipsometer (model M2000V, J.A. Woollam Co.) at 550 nmwavelength. Two first layers (bovine gelatin and TAC) were used for thisexample. The bovine gelatin served as a curl control layer. It was notedthat adhesion of the second layer and the gelatin layer to the TAC layerwas much improved after the heating and stretching. In addition, it isbelieve that such a multilayer compensator as in this example would haveenhanced durability in regards to loss of Rin and Rth after aging such acompensator in conditions such as 1000 hours at 60° C. and 90% relativehumidity.

Notice that in tables A, B, and C a wide variety of Rin and Rth valuescan be obtained by varying the thickness of the second layer and the %extension.

TABLE C % Extension = % Stretch Temp. (° C.) Rth (nm) Rin (nm) 0 roomtemp. −153 2 5 135 −130 22 7 135 −137 34

Table D below shows the impact that % extension and temperature have onout-of-plane and in-plane retardation of a multilayer opticalcompensator (3.6 μm of polymer D on 80 μm of TAC) having positiveout-of-plane birefringence. These retardations are measured with anellipsometer (model M2000V, J.A. Woollam Co.) at 550 nm wavelength.

Poly (N-vinylcarbazole) Polymer D

TABLE D % Extension = % Stretch Temp. (° C.) Rth (nm) Rin (nm) 0 roomtemp. +50 2 5 135 +40 20 7 135 +45 35

Notice in tables B, C, and D that Rth is primarily controlled by thethickness of the second layer, and that Rin is primarily controlled bythe % extension/stretch. Thus, Rth and Rin values can be obtained in anindependently controlled (decoupled) manner.

The techniques described above allow for the manufacture of a multilayercompensators described next. That is, the present invention provides amultilayer compensator comprising one or more polymeric first layers andone or more polymeric second layers, wherein the first layers comprise apolymer having an out-of-plane (Δn_(th)) birefringence not more negativethan −0.01 or not more positive than +0.01, and the second layerscomprise an amorphous polymer having an out-of-plane birefringence morenegative than −0.01 or more positive than +0.01. The overall in-planeretardation (R_(in)) of the multilayer compensator is greater than 20 nmand the out-of-plane retardation (R_(th)) of said multilayer compensatoris more negative than −20 nm or more positive than +20 nm, and whereinthe in-plane retardation (Rin) of said one or more first layers is 30%or less of the overall in-plane retardation (Rin) of said multilayercompensator. Optionally, two or more of the first and said second layersare contiguous.

The first layer is made from polymer film that has an out-of-plane(Δn_(th)) birefringence not more negative than −0.01 or not morepositive than +0.01. Examples of such polymers include:triacetylcellulose (TAC), cellulose diacetate, cellulose acetatebutyrate, polycarbonate, cyclic polyolefin, polystyrene, polyarylatecontaining fluorene groups, and other polymers known to those skilled inthe art.

A combined thickness of the second layers is preferably less thanmicrometers, more preferably from 1.0 to 10 micrometers, and even morepreferably from 2 to 8 micrometers.

The overall in-plane retardation (R_(in)) of the multilayer compensatoris preferably between 21 and 200 nm, more preferably between 25 and 150nm, and even more preferably between 25 and 100 nm.

A combined thickness of the first and second layers is preferably lessthan 200 micrometers, more preferably from 40 to 150 micrometers, andeven more preferably from 80 to 110 micrometers.

In the case where the out-of-plan retardation (R_(th)) of the multilayercompensator is more negative than −20 nm, at least one second layerincludes a polymer containing in the backbone a non-visible chromophoregroup and has a T_(g) above 180° C. The polymer may contain in thebackbone a nonvisible chromophore containing a vinyl, carbonyl, amide,imide, ester, carbonate, aromatic, sulfone, or azo, phenyl, naphthyl,biphenyl, bisphenol, or thiophene group. Examples of polymers suitablefor the second layers include (1) apoly(4,4′-hexafluoroisopropylidene-bisphenol)terephthalate-co-isophthalate, (2) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate,(3) a poly(4,4′-isopropylidene-2,2′,6,6′-tetrachlorobisphenol)terephthalate-co-isophthalate, (4) apoly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenolterephthalate, (5) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′,6,6′-tetrabromo)-bisphenolterephthalate, (6) apoly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol)terephthalate-co-isophthalate, (7) apoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate, or (8) copolymers of any twoor more of the foregoing.

In the case where the out-of-plane retardation (R_(th)) of themultilayer compensator is more positive than +20 nm, at least one secondlayer includes a polymer which contains off the backbone a non-visiblechromophore group and has a glass transition temperature (Tg) above 160°C. The non-visible chromophore group may include a carbonyl, amide,imide, ester, carbonate, phenyl, naphthyl, biphenyl, bisphenol, orthiophene group, or a heterocyclic or carbocyclic aromatic group. Thepolymer of the second layer may contain in the backbone a vinyl,carbonyl, amide, imide, ester, carbonate, aromatic, sulfone, or azogroup. Examples of suitable polymers for the second layer include (A)poly (4 vinylphenol), (B) poly (4 vinylbiphenyl), (C) poly(N-vinylcarbazole), (D) poly(methylcarboxyphenylmethacrylamide), (E)poly[(1-acetylindazol-3-ylcarbonyloxy)ethylene], (F)poly(phthalimidoethylene), (G)poly(4-(1-hydroxy-1-methylpropyl)styrene), (H)poly(2-hydroxymethylstyrene), (I) poly(2-dimethylaminocarbonylstyrene),J) poly(2-phenylaminocarbonylstyrene), (K)poly(3-(4-biphenylyl)styrene), (L) poly(4-(4-biphenylyl)styrene), (M)poly(4-cyanophenyl methacrylate), (N) poly(2,6-dichlorostyrene), (O)poly(perfluorostyrene), (P) poly(2,4-diisopropylstyrene), (O)poly(2,5-diisopropylstyrene), and (and R) poly(2,4,6-trimethylstyrene)or (S) copolymers of any two or more of the foregoing.

Reference will now be made to the drawings in which the various elementsof the present invention will be given numerical designations and inwhich the invention will be discussed so as to enable one skilled in theart to make and use the invention. It is to be understood that elementsnot specifically shown or described may take various forms well known tothose skilled in the art.

FIG. 4A, FIG. 4B and FIG. 4C are elevation schematics of the exemplarymultilayer optical compensators in accordance with the invention whichinclude one or more A polymer layers having an out-of-plane (Δn_(th))birefringence not more negative than −0.01, or not more positive than+0.01, and one or more B amorphous polymer layers having an out-of-planebirefringence more negative than −0.01 or more positive than +0.01.Compensator 401 in FIG. 4A has a structure in which a B layer 409 isdisposed on an A layer 407. The A layer 407 and the B layer 409 arecontiguous. It is also possible to have two B layers 413, 415 disposedon one A layer 411 such as the compensator 403 in FIG. 4B. In other case405, one B layer 417 is sandwiched by two A layers 419, 421. Thecompensator 405 can be formed, for example, by laminating contiguouslayers of A 421 and B 417, and the single layer of A 419. The laminationis done at the interface of B layer 417 and A layer 419, and the twolayers 417 and 419 may or may not be contiguous depending on the methodof the lamination. One skilled in the art could conceive of more complexstructures.

In LCD 501 shown in FIG. 5A, the liquid crystal cell 503 is placedbetween the polarizer 505 and analyzer 507. Transmission axis of thepolarizer 509 and analyzer 511 form angle 90±10° thus, pair of polarizer509 and analyzer 511 are said to be “crossed polarizer”. A multilayeroptical compensator 512 is placed between the polarizer 505 and theliquid crystal cell 503. It can also be placed between the liquidcrystal cell 503 and the analyzer 507. LCD 513 shown schematically inFIG. 5B has two multilayer optical compensators 515, 517 placed on theboth sides of the liquid crystal cell 503. FIG. 5C shows an applicationexample of multilayer optical compensator in a reflective type LCD 519.The liquid crystal cell 503 is located between the polarizer 505 and areflective plate 521. In the figure, the multilayer compensator 523 isplaced between the liquid crystal cell 503 and the polarizer 505.However, it can also be placed between the reflective plate 521 and theliquid crystal cell 503.

Compared to the prior art, embodiments of the present invention avoidretardation increasing agents that could cause undesired coloration orcould diffuse out of the compensator causing retardation loss and/orunwanted chemistry, do not require the use of liquid crystal compoundsand its alignment procedure, provide enhanced optical compensation in arelatively thin (<200 μm) structure, and are easily manufactured.

PARTS LIST

-   101 film-   103 plane of the film-   201 VA liquid crystal cell in OFF state-   203 VA liquid crystal cell in ON state-   205 liquid crystal optic axis-   207 liquid crystal cell substrate-   209 light propagating cell normal direction-   211 light propagating oblique direction-   301 OCB liquid crystal cell in OFF state-   303 OCB liquid crystal cell in ON state-   305 liquid crystal optic axis-   307 cell middle plane-   309 cell boundaries-   401 multilayer optical compensator-   403 multilayer optical compensator-   405 multilayer optical compensator-   407 A layer-   409 B layer-   411 A layer-   413 B layer-   415 B layer-   417 B layer-   419 A layer-   421 A layer-   501 LCD-   503 liquid crystal cell-   505 polarizer-   507 analyzer-   509 transmission axis of polarizer-   511 transmission axis of analyzer-   512 multilayer optical compensator-   513 LCD-   515 multilayer optical compensator-   517 multilayer optical compensator-   519 LCD-   521 reflective plate-   523 multilayer optical compensator-   nx index of refraction in x direction-   ny index of refraction in y direction-   nz index of refraction in z direction-   Δn_(th) out-of-plane birefringence-   Δn_(in) in-plane birefringence-   d thickness of the layer or film-   R_(th) out-of-plane retardation-   R_(in) in-plane retardation-   λ wavelength-   T_(g) glass transition temperature

1. A multilayer compensator comprising one or more polymeric firstlayers and one or more polymeric second layers, wherein: said firstlayers comprise a polymer having an out-of-plane (Δn_(th)) birefringencenot more negative than −0.01 or not more positive than +0.01; saidsecond layers comprise an amorphous polymer having an out-of-planebirefringence more negative than −0.01 or more positive +0.01; whereinthe term “amorphous” means that the polymer does not show long rangeorder by X-ray diffraction and analysis; and the overall in-planeretardation (R_(in)) of said multilayer compensator is greater than 20nm and the out-of-plane retardation (R_(th) ) of said multilayercompensator is more negative than −20 nm in which case at least onesecond layer includes a polymer containing in the backbone a non-visiblechromophore group and has a T_(g) above 180° C. without containing anon-visible chromophore off the backbone or (b) more positive than −20nmin which case at least one second layer includes a polymer whichcontains off the backbone a non-visible chromophore group and has aglass transition temperature (T_(g)) above 160° C., and wherein thein-plane retardation (Rin) of said one or more first layers is 30% orless of the overall in-plane retardation (Rin) of said multilayercompensator.
 2. The multilayer compensator of claim 1 wherein at leasttwo of the layers are contiguous.
 3. The multilayer compensator of claim1 wherein all of said first and said second layers are contiguous. 4.The multilayer compensator of claim 1 wherein the second layers have acombined thickness of less than 30 micrometers.
 5. The multilayercompensator of claim 1 wherein the second layers have a combinedthickness of from 1.0 to 10 micrometers.
 6. The multilayer compensatorof claim 1 wherein the second layers have a combined thickness of from 2to 8 micrometers.
 7. The multilayer compensator of claim 1 wherein theoverall in plane retardation (R_(in)) of said multilayer compensator isbetween 21 and 200 nm.
 8. The multilayer compensator of claim 1 whereinthe overall in-plane retardation (R_(in)) of said multilayer compensatoris between 25 and 150 nm.
 9. The multilayer compensator of claim 1wherein the overall in-plane retardation (R_(in)) of said multilayercompensator is between 25 and 100 nm.
 10. The multilayer compensator ofclaim 1 wherein The combined thickness of the first and second layer isless then 200 micrometers.
 11. The multilayer compensator of claim 1wherein the combined thickness of the first and second layers is from 40to 150 micrometers.
 12. The multilayer compensator of claim 1 whereinthe combined thickness of the first and second layers is from 80 to 110micrometers.
 13. The multilayer compensator of claim 1 wherein theout-of-plane retardation (R_(th)) of said multilayer compensator is morenegative than −20 nm.
 14. The multilayer compensator of claim 13 whereinat least one second layer comprises a polymer containing in the backbonea non-visible chromophore containing a vinyl, carbonyl, amide, imide,ester, carbonate aromatic, sulfate, or azo, phenyl, naphthyl, biphenyl,bisphenol, or thiophene group.
 15. The multilayer compensator of claim13 wherein at least one second layer comprises a copolymers containing(1) a poly(4,4′-hexafluoroisopropylidene-bisphenol)terephthalate-co-isophtha (2) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate,(3) a poly(4,4′-isopropylidene-2,2′6,6′tetrachlorobisphenol)terephthalate-co-isophthalate, (4) apoly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenolterephthalate, (5) apoly(4,4′-hexahydro-4,7-methanoindan5ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′6,6′-tetrabromo)-bisphenolterephthalate, (6) apoly(4,4′-isopropylidene-bisphenol-co-4,4′(2-norbornylidene) bisphenol)terephthalate-co-isophthalate, (7) apoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate, or (8) copolymers of any twoor more of the foregoing.
 16. The multilayer compensator of claim 13wherein at least one first layer contains a polymer that comprisestriacetylcellulose, cellulose diacetate, cellulose acetate butyrate,polycarbonate, cyclic polyolefin or polyarylate containing fluorenegroups.
 17. The multilayer compensator of claim 1 wherein theout-of-plane retardation (R_(th)) of said multilayer compensator is morepositive than +20 nm.
 18. The multilayer compensator of claim 17 whereinat least one second layer includes a polymer which contains off thebackbone a vinyl, carbonyl, amide, imide, ester, carbonate, aromatic,sulfone, or azo group.
 19. The multilayer compensator of claim 17wherein the non-visible chromophore group includes a carbonyl, amide,imide, ester, carbonate, phenyl, naphthyl, biphenyl, bisphenol, orthiophene group.
 20. The multilayer compensator of claim 17 wherein thenon-visible chromophore group includes a heterocyclic or carbocyclicaromatic group.
 21. The multilayer compensator of claim 17 wherein atleast one second layer comprises a polymer selected from the groupconsisting of (A) poly(4 vinylphenol), (B) poly(4 vinylbiphenyl), (C)poly (N-vinylcarbazole), (D) poly(methylcarboxyphenylmethacrylamide),(E)poly[(1-acetylindazol-3-ylcarbonyloxy)ethylene], (F)poly(phthalimidoethylene), (G)poly(4-(1-hydroxy-1-methylpropyl)styrene), (H)poly(2-hydroxymethylstyrene), (I)poly(2-dimethylaminocarbonylstyrene),J)poly(2-phenylaminocarbonylstyrene), (K) poly(3-(4-biphenylyl)styrene),(L) poly(4-(4-biphenylyl)styrene), (M) poly(4-cyanophenyl methacrylate),(N) poly(2,6-dichlorostyrene), (O) poly(perfluorostyrene), (P)poly(2,4-diisopropylstyrene), (Q) poly(2,5-diisopropylstyrene), and (andR) poly(2,4,6-trimethylstyrene) or (S) copolymers of any two or more ofthe foregoing.
 22. The multilayer compensator of claim 21 wherein atleast one first layer contains a polymer that comprisestriacetylcellulose, cellulose diacetate, cellulose acetate butyrate,polycarbonate, cyclic polyolefin, polystyrene or polyarylate containingfluorene groups.
 23. A liquid crystal display comprising a liquidcrystal cell, a pair of crossed polarizers located one on each side ofthe cell, and at least one compensator of claim
 1. 24. The liquidcrystal display of claim 23 wherein said liquid crystal cell is avertically aligned cell, a twisted nematic cell, an in-plane switchingmode cell, or a optically compensated bend liquid crystal cell.
 25. Aliquid crystal display comprising a liquid crystal cell, at least onepolarizer, a reflective plate, and at least one compensator of claim 1.26. The liquid crystal display of claim 25 wherein said liquid crystalcell is a vertically aligned cell, a twisted nematic cell, an in-plane.switching mode cell, or a optically compensated bend liquid crystalcell.
 27. A process for forming a compensator for an LC displaycomprising coating or co-casting one or more second layers that containan amorphous polymer in a solvent onto one or more first layers thatcontain a polymer, and stretching the first layers and second layerssuch that; said first layers comprise the polymer having an out-of-plane(Δn_(th)) birefringence not more negative than −0.01 or not morepositive than +0.01; said second layers comprise the amorphous polymerhaving an out-of-plane birefringence more negative than −0.01 or morepositive than +0.01wherein the term “amorphous” means that the polymerdoes not show long range order by X-ray diffraction analysis; and theoverall in-plane retardation (R_(in)) of said multilayer compensator isgreater than 20mm and the out-of-plane retardation (R_(th)) of saidmultilayer compensator is (a) more negative than −20nm in which case atleast one second layer includes a polymer containing in the backbone anon-visible chromophore group and has a T_(g) above 180° C. withoutcontaining a non-visible chromophore off of the backbone, or (b) morepositive than +20 nm in which case at lease one second layer includes apolymer which contains off the backbone a non-visible chromophore groupand has a glass transition temperature (Tg) above 160° C., and whereinthe in-plane retardation (Rin) of said one or more first layers is 30%or less of the overall in-plane retardation (Rin) of said multilayercompensator.
 28. The process of claim 27, wherein said stretchingincludes restraining at least one side of the first and second layersand drying the first and second layers by application of heat to thefirst and second layer.
 29. The process of claim 27, further comprisingdrying the first and second layers to remove the solvent prior toapplication of heat and then stretching the first and second layers. 30.The process of claim 27 wherein the out-of-plane retardation (R_(th)) ofsaid multilayer compensator is more negative than −20 nm.
 31. Theprocess of claim 30 wherein at least one second layer comprises apolymer containing in the backbone a non-visible chromophore containinga vinyl, carbonyl, amide, imide, ester, carbonate, aromatic, sulfone, orazo, phenyl, naphthyl, biphenyl, bisphenol, or thiophene group.
 32. Theprocess of claim 30 wherein at least one second layer comprises acopolymers containing (1) a poly(4,4′hexafluoroisopropylidene-bisphenol)terephthalate-co-isophthalate, (2) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate,(3) a poly(4,4′-isopropylidene-2,2′6,6′-tetrachlorobisphenol)terephthalate-co-isophthalate, (4) apoly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenolterephthalate, (5) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′6,6′-tetrabromo)-bisphenolterephthalate, (6) apoly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol)terephthalate-co-isophthalate, (7) apoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate, or (8) copolymers of any twoor more of the foregoing.
 33. The process of claim 32 wherein at leastone first layer contains a polymer that comprises triacetylcellulose,cellulose diacetate, cellulose acetate butyrate, polycarbonate, cyclicpolyolefin or polyarylate containing fluorene groups.
 34. The process ofclaim 27 wherein the out-of-plane retardation (R_(th)) of saidmultilayer compensator is more positive than +20 nm.
 35. The process ofclaim 34 at least one second layer includes a polymer which contains offthe backbone a vinyl, carbonyl, amide, imide, ester, carbonate,aromatic, sulfone, or azo group.
 36. The process of claim 34 wherein thenon-visible chromophore group includes a carbonyl, amide, imide, ester,carbonate, phenyl, naphthyl, biphenyl, bisphenol, or thiophene group.37. The process of claim 34 wherein the non-visible chromophore groupincludes a heterocyclic or carbocyclic aromatic group.
 38. The processof claim 34 wherein at least one second layer a polymer selected fromthe group consisting of(A) poly (4 vinylphenol), (B) poly (4vinylbiphenyl), (C) poly (N-vinylcarbazole), (D)poly(methylcarboxyphenylmethacrylamide), (E)poly[(1-acetylindazol-3-ylcarbonyloxy)ethylene], (F)poly(phthalimidoethylene), (G)poly(4-(1-hydroxy-1-methylpropyl)styrene), (H)poly(2-hydroxymethylstyrene), (I) poly(2-dimethylaminocarbonylstyrene),J) poly(2-phenylaminocarbonylstyrene), (K)poly(3-(4-biphenylyl)styrene), (L) poly(4-(4-biphenylyl)styrene), (M)poly(4-cyanophenyl methacrylate), (N) poly(2,6-dichlorostyrene), (O)poly(perfluorostyrene), (P) poly(2,4-diisopropylstyrene), (Q)poly(2,5-diisopropylstyrene), and (and R) poly(2,4,6-trimethylstyrene)or(S) copolymers of any two or more of the foregoing.
 39. The process ofclaim 38 wherein at least one first layer contains a polymer thatcomprise triacetylcellulose, cellulose diacetate, cellulose acetatebutyrate, polycarbonate cyclic polyolefin, polystyrene or polyarylatecontaining fluorene groups.
 40. A liquid crystal display comprising aliquid crystal cell, a pair of crossed polarizers located one on eachside of the cell, and at least one compensator made by the process ofclaim
 27. 41. The liquid crystal display of claim 40 wherein said liquidcrystal cell is a vertically aligned cell, a twisted nematic cell, anin-plane switching mode cell, or a optically compensated bend liquidcrystal cell.
 42. A liquid crystal display comprising a liquid crystalcell, at least one polarizer, a reflective plate, and at least onecompensator made by the process of claim
 27. 43. The liquid crystaldisplay of claim 42 wherein said liquid crystal cell is a verticallyaligned cell, a twisted nematic cell, an in-plane switching mode cell,or an optically compensated bend liquid crystal cell.